Hoist controller



Jan. 29, 1952 Filed June 21, 1945 W. H- ELLIOT HOIST CONTROLLER 2 SI-iEETSSHEE-T l Jan. 29, 1952 w. H. ELLIOT HOIST CONTROLLER 2 SHEETSSHEET 2 Filed June 21, 1945 I 3? m/ H 4% 0 b 2 m w $2- Q N n m y s 02.523 1 Mom 3 533 N /ON o f0 on K Patented Jan. 29, 1952 HOIST CONTROLLER William 1!. Elliot, Shorewood, Wis., allgnor to Cutler-Hammer, Inc., Milwaukee, Wis a corporation of Delaware Application June 21, 1945, Serial No. 600,714

10 Claims.

The invention relates to the speed control of motors and while not limited thereto is particularly adapted to the control of induction motors operating hoists which are required to operate over a relatively wide range of loads and speeds and where it is desired to maintain certain speeds constant for widely varying positive and/or negative motor loads.

In carrying out the invention gaseous discharge devices are connected in various relations to the primary motor winding and their conductivity is controlled in response to the tendencies of the speed or the motor to vary while the operating speed may be manually adjusted.

An object 01' the invention is to provide an alternating current hoist controller aflording a plurality oi constant lowering and/or hoisting speeds.

Another object is to provide a controller of the aforementioned type which aflords lowering of an overhauling load, while limiting the speed of lowering.

Another object is to provide a controller afiording a low constant speed for hoisting a light load, while alternatively afiording hoisting of any load within the capacity of the hoist motor.

Another object is to provide a controller atlording adjustment of the stalled torque of a motor.

Another object is to provide a controller ailording continuous or stepless variation of the lowering speed.

Other objects and advantages will be hereinafter explained.

In the accompanying drawings,

Figure l is a diagram of a hoist controller embodying the invention.

Fig. 2 is a modification oi the system illustrated in Fig. 1 and Fig. 3 is a diagram illustrative of the speedtcrque characteristics of the systems, Figs. 1 and 2.

Referring to Fig. l, the same illustrates an induction motor it, having a three phase primary winding Ill and a three phase second winding II". The primary winding may be supplied with electric energy irom the lines L, L, L or a three phase supply source. One terminal of the primary winding is directly connected to the line L. A second terminal is connectible to the line L', through normally open main contacts ll of an electromagnetic main switch ll, while the third terminal of the primary winding I- is connectible to the line L through normally open main contacts ll" of said electromagnetic switch ll, said contacts ll being in series with normally open main contacts l2 or an electromagnetic switch I2. The electromagnetic switch II is further provided with an energizing winding l| and normally open auxiliary contacts II and li. The electromagnetic switch I2 is further provided with an energizing winding l2", normally open auxiliary contacts l2 and normally closed auxiliary contacts I2 The system also includes an ,electromagnetic switch I; having an energizing winding it, normally open main contacts I3 and normally open auxiliary contacts l3. A pair of electron tubes l4 and it are connected in inverse-parallel with each other and in series with the contacts l3 to parallel the contacts l2. The electron tubes [4 and I5 are provided with cathodes I4 and I5, respectively, anodes I4 and I5", respectively, and control electrodes l4 and IV, respectively. Connected to the terminals oi the three phase secondary winding ill are secondary resistors l6, l1 and I8, respectively. These resistors are divided into three series connected sections it, il and l$; I1, [l and I1; II, l8 and l8, respectively. The end terminals of the resistors IG I1 and it are joined to form a star connection. The resistors l6, I1 and I! may be short circuited by normally open contacts 20 and 20 of an electromagnetic switch 20, which is provided with an energizing winding 20. Similarly the sections Ni Il and I8" may be short circuited by the contacts 2 i and 2t" of an electromagnetic switch 2|, having an energizing winding 2l and the sections i6, ll and ll may be short circuited by normally open contacts 22', 22 of an electromagnetic switch 22 which has an energizing winding 22. Upon closure of the contacts 22 and 22 the secondary winding I." is short circuited as is apparent irom the diagram. One terminal each 01' the windings 20, 2i and 22 is connected through contacts I I to bus bar LP.

The system also includes a drum type master controller M of any suitable type of construction having stationary contacts 23 to 21, inclusive, and 2! to 35, inclusive, which are adapted to make contact with movable contact segments arranged on a rotating cylinder of the master controller, as will be explained hereinafter.

The motor is provided with an electromagnetic brake of any suitable type having an energizing winding 38, one terminal of which is connected to the line L while the second terminal is connected through normally open contacts 31 of an electromagnetic relay 31 and the contact .I I to the bus L. The relay 31 is provided with an energizing winding 31", one terminal of which is connected to bus L while the second terminal is connected to the contact 24' of the master drum. A low voltage relay 86 has an energizing winding 86", one terminal oi! which is connected to the bus L, whfle the second terminal is connected to the contact 25 of the master drum. The relay 86 is also provided with normally open contacts 86 and 86. One terminal of the contacts 86 is connected to the bus L and also to the stationary contact 28, while the other terminal is connected to the contact 25. The normally open contacts 86 have one terminal connected to the bus L while the other terminal is connected to one terminal each of the windings il, I2 and [8. The second terminal of the winding l8 is connected to the contact 82 of the master drum and also through the contacts l2 to drum contact 21.

The second terminal of winding 12* is connected to the drum contact 26. The second terminal of the winding ii is connected through contacts i8 to drum contact 21 and alternatively through contacts l2 to contact 26.

To control the grid potential of the tubes i4 and I5, respectively, secondary windings 89" and 89 of a transformer 89 having a primary winding 89 are connected between the respective cathodes and control electrodes of said tubes [4 and i5. One terminal of winding 39 is connected to the center tap or an inductance 40 connected across lines L1 and In. Connected between the other terminal of the winding 89 and one end terminal of the inductance 40 is a resistor 4|, while connected between the last mentioned terminal of winding 39 and the other end terminal of the inductance 40 are the secondary windings 42' 01' a saturable reactor 42 which is provided with a saturating winding 42. Thus, the inductance 40, the winding89, the resistor 4| and the windings 42 of the saturable reactor 42 form a phase shifting network for ad- Justing the phase of the potentials impressed upon the control electrodes l4 and I5 by the windings 86 and 89 The energization of the winding 42 is con-- trollable in accordance with the speed of the motor armature by the apparatus which will now be described. Coupled to the shaft of the armature Iii is an alternating current tachometer generator 48, which supplies alternating current energy to a full wave rectifier 44 of any suitable type. One of the direct current terminals of said rectifier is connected to the contact 84 of the master controller. The other direct current terminal is connected to the control electrode 45 of an electron tube 45, which is also provided with a cathode 45 and an anode 45. A smoothing condenser 44 may be connected across the direct current terminals of the rectifier. The cathode 45 is connected to the zero potential bus bar 46 of a direct current power supply which is also provided with bus bars 41 and 46 having potentials positive with respect to the potential of the bus bar 46, the potential of the bus bar 46 being of the order of twice the potential of the bus bar 41. The anode of the tube 45 is connected through a resistor 49 to the bus bar 41, and is also connected to the control electrode 56 of an electron tube 50, which is also provided with a cathode 50 and an anode 50 The cathode 56' is connected to the movable contact of a voltage divider 5| connected across the bus bars 46 and 41. The anode 50 is connected to the bus bar 41-, through a resistor 52. Also connected to the bus bar 41 is the cathode 58' of an electron tube 58 which is also provided with an anode 58 and a control electrode 58. The anode 58" is connected through the winding 42" to the bus bar 46. Normally open contacts 54 of an electromagnetic relay 54 are interposed between the anode 50 and the control electrode 58 and normally closed contacts 54 of said relay 54, are interposed between the control electrode 58 and the anode 45 The relay 54 is provided with an energizing winding H, one terminal of which is connectable through the contacts il to the line I), while the second terminal is connected to the contact 82 of the master controller and also to one terminal of the winding l8 and one terminal oi! contacts l2. The other terminal of contacts I2 is connected to one terminal of contacts I8 and to contact 21. The bus bar 41 is connected to drum contact 85. A variable resistor 55 is connected between the segments associated with drum contacts 84 and 85 in such a manner that upon rotation of the master controller the contact 84 makes circuit with difterent parts or said resistor. The bus bar 46 is connected through a resistor 56 to drum contact 88. The resistors 55 and 56 form a voltage divider across the bus bars 46 and 41 for controlling the potential impressed upon the control electrode 45 as will be explained hereinafter.

All of the segments which cooperate with the stationary contacts 28 to 21, inclusive, and 28 to 82, inclusive, of the master controller are connected together, while the segments which cooperate with the stationary contacts 88, 84 and 85 are separately mounted and insulated from the former segments. Furthermore, the segment cooperating with the contact 88 is directly connected to the segment cooperating with the contact 84, while, as aiorestated, the segments cooperating with the contacts 84 and 85 are connected together through the resistor 55. The master controller may be provided with a conventional starwheel to locate the drum in the hoisting positions 1 to 5, inclusive, the center position and the first lowering position. For stepless control of lowering, the controller may, however, be arrested at any point between the first and the last lowering position so as to vary the potential derived from the resistor 55 in a substantially stepless manner.

The operation of the system thus far described is as follows: When the lines L L" and L are energized, a circuit is established from line L to drum contact 28, through the cooperating segments to contact 25, through winding 86 to line L Thereupon the relay 86 is energized to close the contacts 86 and 86. Closure of contact 8i! parallels the connection established between the contacts 28 and 25 of the master drum, So that now the drum may be moved from the off position to the first hoist position, thereby interrupting the circuit at contact 25. but the relay 86 remains energized over the parallel circuit heretofore traced.

When the master drum is in thefirst hoisting position a circuit is closed from line L over contact 24, through the winding 81 to line L, and the relay 31 is energized thereby closing the contacts 81- to energize the brake winding 86 and release the brake when the contact ii is closed as hereinafter set forth.

As the contact 86 is closed a circuit is also established from line L, through contacts 86. winding I8, contacts l2 to contact 21, back to line L. This efiects energization of switch l8 to .close the contacts i8 and also the contacts l8.

Thereupon a further circuit is established from line I), through contacts 33, winding ll, contacts l3, drum contact 21, and back to line L Energization ofwinding I causes closure of contacts Il and I I, thereby connecting the primary winding Il to the bus bar L. Closure of contact li completes a circuit from bus bar L through contacts II and I3", through the tubes l4 and I5 to the third terminal of winding Ill. The motor is .thus energized to receive an unbalanced polyphase current in accordance with the conduction of tubes l4 and Ii. At the same time the brake solenoid winding 36 is energized through closure of contact I! and the brake is lifted and the motor starts if the load is not too heavy.

A circuit is also established from line L through drumcontacts 29, the energizing winding 20, over contact ll to line L so that the resistors l6", l1 and it are short circuited and the motor starts with the remainder of the balanced secondary resistance star connected across the secondary winding. A further circuit is established from line LP, through contacts ll", winding 54', to drum contacts 32, and through the drum to line L This energizes the relay 54 thereby opening contacts 54 and closing contacts 54'. V

The phase shifting network, comprising the transformer 39, inductance 40, resistor 4| and reactor coil 42*, is normally so adjusted that with no current flowing in the saturating winding 42, the voltage impressed on the control electrodes l4 and I5, respectively, has a maximum phase displacement with respect to the cathode voltages so that little current flows through the tubes to the primary motor winding. However, the potential of grid 4.'i is positive, so that a current flows through resistor 49, which makes the gri 50 negative and tube 50 accordingly carries little or no current. The voltage drop through resistor 52 is therefore practically zero and the grid 53 has practically zero potential, rendering the tube 53 fully conducting and thereby saturating the reactor 42, which in turn shifts the voltage of rids l'4 and Ill to render the tubes l4 and I fully conducting. The motor therefore starts and the tachometer generator 43 generates a voltage which is rectified and the rectified voltage, which is opposed to the voltage drop through resistor 58 reduces the potential of the control electrode 45. The total potential impressed upon the control electrode 45 with respect to the cathode 45" is composed of the voltage drop through the resistor 56 and the rectified voltage of the tachometer 43. This potential renders the tube 45 conducting and the higher its conduction the lower will be the potential of its anode and therefore the potential impressed upon the control electrode 50. The potential of the control electrode 50 with respect to the cathode 50' is the voltage drop through the tube 45 minus the voltage drop between the bus bar 46 and the movable contact Ii. This potential may be varied by adjustment of contact 5|. The potential of the anode ill is higher than the potential of the cathode by the voltage drop through the tube 50. The potential oi the anode 50 is impressed through the contact 54" upon the grid 53, thereby regulating the current flowing from the bus bar 41, through the tube 53, the saturating winding 42, back to bus bar 43. It will be observed that the potential impressed upon the grid 53 becomes more negative, the higher the current flow is through the tube ill by virtue of the voltage drop through the resistor 32. Hence as the speed of the motor increases and the voltage of the tachometer 43 increases, the conduction of tube 45 decreases, which in turn increases the conduction of the tube 50 and this decreases the conduction of the tube 53, thereby decreasing the saturating current of the winding 42. The phase relation of the potentials of the grids l4 and IS with respect to the cathodes is such that a decrease of the current of the saturating winding 42 decreases the conduction of the tubes l4 and I5. Therefore as the tachometer 43 speeds up the decrease through the tube 63 decreases the saturating current, thereby tending to reduce the current flow through the tubes 14 and 15 to maintain it at a value which will maintain the desired speed of themotor. This speed is determined by the magnitude of the voltage drop across resistor 56.

Thus the present system provides by control of the unbalance of primary current, a low, constant hoisting speed at all loads between zero load and a maximum load determined by the resistance in the secondary circuit of the motor.

If it is desired to increase the hoisting speed, the master controller is moved to the second position. This deenergizes the electromagnetic switch I3 and energizes the electromagnetic switch l2, by a circuit from line L1, through drum contacts 23 and 26, winding 12, contacts 33, to line L Closure of contacts l2 short circuits the tubes l4 and I5 and opening of contacts l3 disconnects the tubes from the line L. The motor now operates with balanced primary current in the usual manner. At the same time relay 20 is deenergized thereby connecting all of the secondary resistors across the motor secondary. The relay 54 is also deenergized and the electronic speed regulating circuit has no more control over the motor.

To further increase the speed of the motor in the hoisting direction, the master controller is moved successively to the third, fourth, and fifth position in which the relays 20, 2|, and 22, respectively, are energized; thus gradually cutting out sections of the secondary resistor and ultimately short circuiting the secondary winding of the motor whereupon the motor operates at its maximum speed in the hoisting direction.

The speed characteristic of the motor for the different positions of the master drum in the hoisting position is illustrated in the diagram, Fig. 3. As has been explained, in the first hoisting position one section of secondary resistance in all three phases is short circuited by the switch 20 and the balance of the secondary resistance is such that with the tubes l4 and I5 fully conductive, the stalled torque of the motor is of the order of 200 per cent and the speed-torque of the motor is therefore represented by the curve 11 However, due to the action of the tachometer 43, the conductivity of the tubes is regulated in such a manner that the speed of the motor is maintained at a constant value indicated by the horizontal line H except when the torque required of the motor exceeds a given value, whereupon the speed torque curve H merges into the line H On the second hoisting point, the tubes l4 and I5 are cut out of circuit and all of the secondary resistors are inserted. The total secondary resistance in the present instance is chosen so that the corresponding stalled torque of the motor is per cent. The speed of the motor therefore varies with the load in accordance with curve H. On the third speed point.

the value of the secondary resistance is the same as it was on the first speed point. Therefore, the speed of the motor varies in accordance with the curve H The curve H represents the speedtorque curve on the fourth point of the controller, while the curve H represents the maximum speed-torque curve obtaining when the secondary motor winding i is short circuited.

If it is desired to lower a load, the motor controller is moved to the position I in the lowering direction. As will be apparent in this and in allother lowering positions, the electromagnetic switches II and I3 are energized while the electi ornagnetic switch I2 is deenergized. The switch 20 being deenergized, the basic speed-torque curve of the motor for all lowering positions of the master drum is therefore represented by the curve H However, due to the action of the tachometer 43 upon the control electrodes of the tubes l4 and I in the manner described, the tubes are not fully conducting during the respective half-cycles and therefore the primary winding HP is supplied with an unbalanced voltage and the current passing through the tubes to the motor is regulated to a value to afford a speed torque curve represented by the horizontal line U in Fig. 3. In other words, the speed is maintained constant for all torques up to that value at which the line U intersects the speed torque curve H.

It it is now desired to increase the lowering speed of the motor, the master controller is moved in the lowering direction so as to gradually move the contact 34 with respect to resistor 55. The

eilect of this is to render the control electrode 45 progressively more positive with respect to its cathode and thus increase the current passing through the tube 45.

In' the lowering direction, the relay 54 is not energized. Therefore, the contacts 54 are closed and the contacts 54 are opened with the result that the voltage of the anode 45 is impressed on the grid 53. Hence an increase in the potential impressed upon the grid 45, due to the movement of resistor 55 with respect to contact 34, resulting from rotation of the master controller, causes a decrease of the conduction of the tube 53 and therefore a decrease in the conduction of the tubes i4 and i5 and an increase in the speed of the motor. Furthermore, an increase in the speed of the tachometer 43 results in a decrease of the positive potential of the grid 45, which causes an increase of the conduction of the tube 53 and an increase of the conduction of the tubes l4 and [5, which will lower the lowering speed of the motor.

By progressively increasing the potential of contact 34 by rotation of the master controller in the lowering direction, speed torque curves similar to those marked U, U U and U in Fig. 3 may be obtained. The maximum torque in each case is determined by the intersection of the horizontal lines with speed-torque curve II.

It is well known that the torque of a three phase motor supplied with unbalanced primary current is very low when the unbalancing is great and the speed of the motor is low and that the motor has to be brought up to a certain minimum speed before any substantial torque can be developed thereby. This is indicated by the curve S in Fig.

, 3. The minimum speed at which the motor operating single phase, that is, with zero conduction of the tubes 14 and I5, can develop any torque is shown to be approximately 75 per cent of synchronous speed. If this speed, which in the pres- 8 ent case is the lowering speed of the motor, increases, the minimum torque increases as shown by curve 8.

In the control shown by Fig. l, provision was made for maintaining only a single hoisting speed constant, which speed is represented by the curve H It would, of course, be possible to provide for a plurality of constant hoisting speeds in a manner similar to that for obtaining a plurality of constant lowering speeds. This could be accomplished by providing a separate resistor in place of the resistor for hoisting, or by providing the master controller M with means for commi' Eating the resistor 55 for hoisting in a manner similar to the manner in which it is commutated for lowering.

The controller shown and described does not afford any positive lowering torque, which is sometimes required to lower a crane hook against the friction of the mechanical system and/or an overbalancing counterweight. This, however, can be provided for by the addition of a conventional reversing switch for reversing two of the primary phases of the motor thus reversing the torque of the motor with the master controller in an added position preceding the first lowering position in the drawing.

The system disclosed in Fig. 2 is a modification of the system shown in Fig. 1 and provides for energizing the primary winding H! of the motor under certain lowering conditions with single phase current. The system, Fig. 2, is the same as that of Fig. 1, except for the following changes: The segment of the master controller which engages the stationary contact 21 in the lowering direction is of such length that it engages the stationary contact 21 only in the first and second lowering position of the master drum. A stationary contact 28 and a cooperating segment have been added to the master controller, the segment engaging the contact 28 in the third to the fifth lowering position. An additional segment has been added which engages the stationary contact finger 29 in the third to the fifth lowering position. The electromagnetic switches l2 and I3 are provided with additional normally closed auxiliary contacts [2 and I3", respectively. There is also added an electromagnetic switch 60, having an energizing winding 80', nor- .r ally open main contacts and 60 and normally closed auxiliary contacts 50. The auxiliary contacts 50 are interposed between one terminal of the winding 13' and the drum contact 32, while the contacts parallel the contacts IS". The auxiliary contacts l2 are adapted to connect the drum contact 28 with one terminal of the energizing winding 60'. The second terminal of the winding 60' is connected through normally closed contact 13 and the normally open contacts 58 to the bus bar 1;. A resistor ii is connectible between that terminal of the winding Iii which is energized from the line D when the switch l I is closed, and the anode I4 through the contacts 60". If desired, under certain conditions the resistor may alternatively be connected to that terminal of the motor winding Ill which is connected to the line L, as shown in dotted lines.

The operation of the system shown in Fig. 2 for hoisting. is the same as that of the system shown in Fig. 1 and described in connection therewith. If it is desired to lower a load with the controller, Fig. 2, the master controller is moved to the first lowering position. In this position the electromagnetic switches l2 and 50 are deenergized and the electromagnetic switches ii and I; are energized. The motor primary is thus supplied with polyphase current through the tubes l4 and I5 and the operation is the same as in the system, Fig. 1, in the first lowering position. By moving the master controller gradually toward the second lowering position, the resistance 55 is commutated by contact 34 resulting in an action similar to that described in connection with Fig. 1.

Now, by moving the drum from the second toward the third position, the switch I! is deenergized by interrupting its energizing circuit at the contact 21, while the electromagnetic stitches 60 and 20 are energized by closing the circuit in the master controller at the contacts 28 and 29, respectively. Thus the motor primary winding i is energized single phase. The basic speed torque curve for the three phase primary motor connection is represented by the curve 1-! in Fig. 3. If instead of supplying polyphase current to the primary winding, two of the terminals are joined together and the winding is supplied with single phase current, a speed torque curve substantially parallel to curve H, the slope of which depends upon the resistance in the secondary circuit, results. By inserting resistance between the two phase windings which are connected together, this curve, D is in turn shifted to the left as represented by the curve D or to the right as represented by the curve D in Fig. 3, the direction of shift being dependent upon the phase rotation and sequence of the three-phase supply. The shift can be changed for a given installation by changing the connection of the resistor 6| to that shown in dotted lines, thus making it possible to influence the speed torque curve for lowering. The amount of shift in either direction may be varied by varying the value of the resistor 6|. The higher that resistor up to a certain value the greater will be the shift of the curves D or D from the curve D However, due to the action of the tachometer 43 on the conductivity of the tubes l4 and IS, the speed-torque curves are modified so as to maintain the speed of the motor substantially constant as represented by the horizontal lines U U, U between the limiting speed-torque curves,S and D D or D, corresponding to the positions of the master controller indicated in the diagram. The minimum torque which the motor may exert at any speed is again represented by the curve S in Fig. 3. It will be observed that speed-torque curve D provides a certain amount of power lowering torque for an empty hook, whereas D provides a certain amount of positive torque for taking up slack in the hoisting cable, without providing sufilcient torque to hoist the load and provided the tubes are rendered conducting while the motor is at rest.

I claim:

1. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase primary winding includin a plurality of terminals for connecting it to said source. space discharge means having a control element for varying the effective impedance thereof interposed between said source and one of said terminals. means to impress upon said control element a biasing potential variable relative to the voltage of said source in response to speed variations of said motor, and means to reverse the sense of said response to thereby control the current passing through said first named 10 a single terminal of said primary winding, motor speed control through a range extending between approximately and 250% slip.

2. In a hoist control system or the like having a motor subject to an overhauling load, in combination, a source of polyphase current, a motor having a polyphase primary winding including a plurality of terminals for connecting it to said source, an impedance, space discharge means connected to one of said motor terminals and having a control element for varying the effective impedance thereof, means for interposing said space discharge means between said source and said one terminal and alternatively disconnecting said space discharge means from said source and connecting it in series with said impedance between said one and another of said terminals, and means to impress upon said control element a biasing potential variable relative to the voltage of said source in response to speed variations of said motor, to thereby control the current passing through said first named means to afford motor speed control through potential regulation of the terminals of said primary winding.

3. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase primary winding including a plurality of terminals for connecting it to said source, a space discharge device interposed between said source and one of said terminals and having'a control element fOr varying its effective impedance in response to variations of a potential impressed thereon, means for impressing upon said control element an alternating potential of the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor, means to reverse the sense of said response, and means to derive a modified voltage from said speed responsive potential and impress it upon said first mentioned means, to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a range extending between approximately 90% and 250% slip.

4. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase primary windin including a plurality of terminals for connecting it to said source, a pair of space discharge devices connected inversely parallel to each other and interposed between said source and one of said terminals, each having a control electrode, means for impressing upon said control electrodes alternating potentials of the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor, manual means to reverse the sense of said potential variations relative to the variation of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a range extending between approximately 90% and 250% slip.

5. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase winding including a plurality of terminals for connecting it to said source, a pair of space discharge devices connected inversely parallel to each other and each having a control electrode, means for impressing upon said control electrodes alternating potentials of the lneans to afford, through potential regulation of 7 same frequency as that of said polyphase current,

means affording a potential which varies in response to the speed of said motor, manual means for effecting connection of said terminals to said source and interposing said space discharge device between said source and one of said terminals, said manual means including means affording adjustment of said speed responsive potential with respect to said speed and further including selective means affording reversal of said potential variations relative to the variations of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary, winding, motor speed control through a range extending between approximately 90% and 250% slip.

6. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase winding including a plurality of terminals for connecting it to said source, a pair of space discharge devices connected inversely parallel to each other and each having a control electrode, means for impressing upon said control electrodes alternating potentials of the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor, manual means operative to effect connection of said terminals to said source and interposing said space discharge devices between said source and one of said terminals, said manual means including means affording adjustment of said speed responsive potential with respect to said speed and including means to reverse the relation of said potential variations and the variations of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means, to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a range extending between approximately 90% and 250% slip, said manual means being alternatively operative to eifect connection of said terminals to said source exclusive of said space discharge devices to afford operation of said motor through a range between approximately 100% and substantially zero slip.

7. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase winding including a plurality of terminals for connecting it to said source, an impedance, a pair .of space discharge devices connected inversely parallel to each other and each having a control electrode, means for impressing upon said control electrodes alternating potentials of the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor, manual means for effecting connection of said terminals to said source and interposing said space discharge device between said source and one of said terminals and alternatively effecting disconnection of said one terminal from said source and connection thereof in series with said space discharge devices to another of said terminals, said manual means including means affording adjustment of said speed responsive potential with respect to said speed and means to reverse the response of said potential variations relative to the variations of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary winding,

bination, a source of polyphase current, a motor having a polyphase winding including a plurality of terminals for connecting it to said source, an impedance, 9. pair of space discharge devices connected inversely parallel to each other and eaca having a control electrode, means for impressing upon said control electrodes alternating potentials of the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor. manual means operative to effect connection of said terminals to said source and interposing said space discharge devices between said source and one of said terminals and alternatively effecting disconnection of said one terminal from said source and connection thereof in series with said space discharge devices to another of said terminals, said manual means including means affording adjustment of said speed responsive potential with respect to said speed and including means to reverse the response of said potential variations relative to the variations of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a, range extending between approximately and 250% slip, said manual means being alternatively operative to effect connection of said terminals to said source exclusive of said space discharge devices to atford operation of said motor through a range between approximately and substantially zero slip,

"9. In a hoist control system or the like, in combination, a source of polyphase current, a motor having a polyphase primary winding including a plurality of terminals for connecting it to said source and a secondary winding, a variable impedance connected to said secondary winding, a pair of space discharge devices connected inversely parallel to each other and each having a control electrode, means for impressing upon said control electrodes alternating potentials oi the same frequency as that of said polyphase current, means affording a potential which varies in response to the speed of said motor, manual means operative selectively to effect connection of said terminals to said source with and without said space discharge devices between said sourceand one of said terminals, said manual means including means affording adjustment of said speedresponsive potential with respect to said speed and including means to reverse the relation of said potential variations and the variations of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a range extending between approximately 90% and 250% slip, said manual means being alternatively operative to effect connection of said terminals to said source exclusive of said space discharge devices and to vary said variable impedance to afford operation of said motor through a range between approximately 100% and substantially zero slip.

10. combination, a source of polyphase current, a motor having a polyphase primary winding including a plurality of terminals for connecting In a hoist control system or the like, in

it to said source, and a secondary winding, a variable impedance connected to said secondary winding, a second impedance, a pair of space discharge devices connected inversely parallel to each other and each having a control electrode.

means for impressing upon said control elec-.

trodes alternating potentials of the same frequency as that of said polyphase current, means afl'ording a potential which varies in response to the speed of said motor, manual means operative selectively for effecting connection of said terminals to said source with and-without said space discharge devices between said source and one 01 said terminals and alternatively effecting disconnection of said one terminal from said source and connection thereof in series with said space discharge devices and said second impedance to another of said terminals, said manual means including means afiording adjustment of said speed responsive potential with respect to said speed and means to reverse the relation of said potential variations and the variatlons of said speed, and means to impress a voltage derived from said speed responsive potential upon said first mentioned means to afford, through potential regulation of a single terminal of said primary winding, motor speed control through a range extending between approximately and 250% slip, said manual means being alternatively operative to effect connection of said terminals to said source exclusive of said space discharge devices and to vary said variable impedance to afford operation of said motor through a range between approximately and substantially'zero slip. I

WILLIAM H. ELLIOT.

REFERENCES CITED UNITED STATES PATENTS Name Date Meyer May 3, 1921 Number 

